Known photoconductive substances to be used in electrophotographic photoreceptors include various organic and inorganic photoconductive substances. Organic photoconductive substances hold advantages, such as satisfactory film-forming properties, capability of providing a transparent and flexible film, and low cost. However, they are inferior to inorganic photoconductive substances in sensitivity and durability. In case of using an organic photoconductive substance, therefore, it has been proposed to improve sensitivity and durability by using a so-called separate function, lamination type electrophotographic photoreceptor composed of a charge generating layer and a charge transporting layer.
In general, the state-of-the-art electrophotographic photoreceptors are known to have any of disadvantages, such as (1) poor stability of image contrast against repeated use or environmental change, (2) liability to image defects called white spot, black spot, coarse image, pinhole, etc., and (3) insufficient durability due to low adhesive strength between a substrate and a photosensitive layer, causing separation of the photosensitive layer during use.
In order to eliminate these disadvantages, it has been proposed to provide a subbing layer comprising a resin between a substrate and a photosensitive layer. Included in known resins for the subbing layer are poly-p-xylene, casein, polyvinyl alcohol, phenolic resins, polyvinyl acetal resins, melamine resins, nitrocellulose, ethylene-acrylic acid copolymers, polyamide (e.g., nylon 6, nylon 66, nylon 610, copolymer nylon, alkoxymethylated nylon), polyurethane, gelatin, polyvinyl pyrrolidone, polyvinyl pyridine, and polyvinyl methyl ether.
It has also been proposed to form a subbing layer using organozirconium compounds, e.g., zirconium chelate compounds and zirconium alkoxides, or silane coupling agents as disclosed in Japanese patent application No. 2-287232.
What is aimed at by providing a resin layer as a subbing layer is to control volume resistance at such a low level that does not deteriorate electrophotographic characteristics by chiefly using a resin having a relatively large content of a polar group. Since volume resistance of a resin has character of being dependent on ionic conduction, it is considerably influenced by temperature and humidity. That is, when a photoreceptor is exposed to a low temperature and low humidity condition or a high temperature and high humidity condition, the resin layer has markedly increased resistance, leading to deterioration of electrophotographic characteristics of the photosensitive layer, or markedly decreased resistance, leading to loss of functions expected, respectively.
Thus, it was only part of the above-described disadvantages associated with a photoreceptor that has been improved by the conventionally known resin layer. Susceptibility to environmental influences being taken into consideration, the effects of the resin layer are reduced by half. Therefore, the conventional resin layers have been extremely insufficient from the technical consideration.
Where an organozirconium compound (e.g., a zirconium chelate compound or a zirconium alkoxide) or a silane coupling agent is employed either alone or in combination with a binder resin, and particularly where a photoreceptor is under a low temperature and low humidity condition, the volume resistance increases due to evaporation of the water content having been adsorbed into the resin layer, and the development contrast decreases with an increase in residual potential. This easily causes image defects, such as white spot, black spot, coarse image, and pinhole.
In this connection, JP-A-64-44450 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") teaches incorporation of a wetting agent comprising a polyhydric alcohol into a charge generating layer or a subbing layer so as to prevent the layer from increasing its volume resistance even at a low humidity, and JP-A-3-23464 proposes incorporation of polyethylene glycol into a subbing layer. Nevertheless, the water retaining properties of polyhydric alcohols or polyethylene glycol are not enough to sufficiently prevent an increase of volume resistance of the subbing layer when continuously used under a low humidity condition.
On the other hand, where a photoreceptor is used in a laser printer in which an electrostatic latent image is formed using coherent light, such as a semi-conductor laser, as a light source, an interference fringe appears on the printed image due to the interference between the reflected light on the surface of the photoreceptor and that on the surface of the substrate. This can be avoided by roughening the surface of the substrate to thereby reduce the reflection on the substrate. In this case, if a subbing layer formed on the roughened surface of the substrate has a small thickness, it is very likely that charges are injected through the uneven subbing layer to cause image defects, such as black spots or white spots. Therefore, in using a surface-grained substrate, the thickness of the subbing layer to be formed thereon must have its thickness relatively increased by addition of a resin component.
However, where a subbing layer having a relatively large thickness contains an organic chelate compound as described above, it would have an increased content of a reaction residue resulting from the organic chelate compound (mostly an oxygen bond), which gives rise to the problem that the volume resistance of the subbing layer tends to increase especially under a low humidity condition.
A known countermeasure to overcome the problem associated with the use of an organic chelate compound in a thick subbing layer is to allow a photoreceptor produced to stand at a given humidity for several hours to several days thereby to decrease the reaction residue in the subbing layer and to stabilize the characteristics of the photoreceptor.